On all control line models the cg is located in the first 1/3 of the wing. The closer to that point the more maneuverable the model becomes and the harder to fly. Crossing that point makes the plane to hard to control.
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For older airplanes with relatively short tail moments, and small tail feathers, a good starting point is no more than 15% MAC back from the leading edge. On 1/2A airplanes, a little more forward is a good idea.

Not sure how close the Shoestring is to the Buster, but my Buster had 2.25 inches from the leading edge as the GC and she was tail heavy. Could not get it to groove. Was always searching up and down. Set it at about an inch and it flew very well.

1" back from the leading edge is a good starting point on most of these old designs without flaps. You can move it back a little at a time from that point until it starts to float on landing, and then move it back forward to suit you. Generally 10 to 15% of mean air cord like Jim T said is pretty close. Control line planes need to balance quite a bit forward from what R/C planes do.

The CG set should be set in relationship to your particular flying style and expertise level.
A forward CG will have the plane track very nice and it is very docile. This is a good CG for beginners who want to fly level, inverted, and do loops and of course it is excellent if you want to flip the plane over when you land.
Once you are bored of loops it is time to switch to a more rearward CG. At his new CG placement the model will want to turn faster and will start to corner a little better but you will sure feel the difference at the handle because now the plane is feistier. This is where an adjustable handle is very practical. Because the model becomes much more sensitive you need to tighten the spacing in the handle to cool off the plane and maintain the responsiveness of the control to match your feel (I am assuming you already set your handle for the safe forward CG). Perhaps you may want to add a little more sensitivity from the handle to be able to corner the plane. The plane should behave the way you want it to behave, not the other way around. This is where you my stay for a while till you get good feel for the plane and build up your cornering and maneuvering around the pattern skills etc. A light plane is very well served by this CG position.
Some of us go a bit farther than that and push the CG even farther back. Where is the max back position? Well if the model begins to hunt, (unstable in level flight, which means you constantly have to impute to have the model to fly level flight) then you went too far back and you need to back off of it and return the CG little more forward. This is where I like to have my plane set and it is my personal preference. The model will have the smallest corner in this position and because there is the least amount of deflection of controls and you maintain the maximum momentum (it does not slow down too much in the corner) which allows for a safe run speed to the next corner. But expect lighter line tension. In this position the model has maximum response and minimum deflection on controls. A heavy plane is best set this way.
There was one day when I flew my friendís plane and it was not cornering if my life depended on it so we added some tail weight to the model and towards the end of the session the model was behaving quite nicely. We had to close the handle spacing in conjunction with the CG back but it was behaving very well. We both left smiling and it was an eye opening experience for my friend. We only put a few flights (5-6) half fuel tank each to check for the feel and the model was transformed from a lazy dog to a snappy cricket.
As a side note as the CG moves also the lead outs need to follow suit. I cannot emphasize how important is to be able to tune your model.
Have fun experimenting with your CG